Severe thunderstorms have pummelled Sydney's west and southwest as well as large stretches of NSW's north and northeast.
A trough in western NSW is meeting moist and humid air in the state's east to drive the weather pattern, according to the Bureau of Meteorology, with dour weather expected for most of the coming week.
"We have a trough in the western part of the state and in front of that, in the eastern part, we have a lot of warm air, a lot of humidity, and that's all leading to a lot of thunderstorm activity," meteorologist Helen Reid said.
"We do have a nice amount of moisture still coming through from the tropics."
Ms Reid said Sydney would be heavily affected by storms on Monday before settling into a pattern of general rainfall in the coming days.
However, storms were likely to persist across the state's north and northeast.
On Monday afternoon, gusts of almost 80km/h were recorded in Penrith in western Sydney and more than 90km/h in Maitland in the NSW Hunter.
The storm cell hitting Penrith is moving west and likely to reach the Sydney CBD by about 5:30pm on Monday, the bureau said.
Those living in affected areas have been encouraged to move their car undercover or away from trees, secure loose items, keep eight metres away from fallen power lines and avoid driving through flood water.
